1. Field of the Invention
This invention relates to a chemically amplified resist composition and more specifically, to a chemically amplified resist composition improved in the depth of focus.
2. Description of the Prior Art
In the fields of the fabrication of various devices typified by a semiconductor device, which require fine processing on the order of submicrons, there is an increasing demand for actualizing higher densification and higher integration. Under such situations, the requirements for photolithography have become severer.
In recent days, a chemically amplified resist attracts attentions in such fields. This chemically amplified resist makes use of the catalytic action of an acid formed by exposure to light. It is characterized in that since the generation efficiency of an acid is high even under the conditions providing only small exposure energy, it has high sensitivity and high resolution.
The resist is composed principally of a photoacid generator which releases an acid and an acid sensitive resin which undergoes a marked change in the solubility in an aqueous alkaline solution, which is a developer, owing to the generation of the acid.
As an example of the prior art, a chemically amplified resist composition comprising, as an photoacid generator, N-(p-toluenesulfonyloxy)-5-norbornene-2,3-dicarboxyimide and, as an acid sensitive resin, a (hydroxystyrene)-(tert-butylcarboxystyrene) copolymer can be mentioned.
When the above-described resist is exposed to light, first-stage reaction occurs in accordance with the below-described reaction scheme (6), whereby N-(p-toluenesulfonyloxy)-5-norbornene-2,3-dicarboxyimide used as a photoacid generator is decomposed and p-toluenesulfonic acid (p-toluenesulfonic acid ion) which is an acid component, is released. 
Then, in the second-stage reaction, the acid thus released acts on the acid sensitive resin, that is, a (hydroxystyrene)-(tert-butylcarboxystyrene) copolymer, whereby the acid sensitive resin is converted into an alkali-soluble (hydroxystyrene)-(tert-carboxystyrene) copolymer in accordance with the below-described reaction scheme (7). This alkali-soluble resin is dissolved in an alkali developer, whereby development is effected. 
The alkali development is allowed to proceed through the reaction procedures as described above. Owing to a high generation efficiency of an acid in exposed regions, a pattern of high resolution is available.
Owing to the recent tendency to higher densification and higher integration, it has come to be impossible to sufficiently satisfy the request for ultra-fine processing of a device even by using such a resist.
For example, when the above-described resist was applied to a silicon substrate, followed by exposure to light and development by a KrF stepper under the optical conditions of NA of 0.60 and "sgr" of 0.75 to form a contact hole pattern of 0.20 xcexcm, the depth of focus thus obtained was only 0.60 xcexcm. It was therefore difficult to form a contact hole having a sufficient rectangularity.
On the surface of a wafer, there exists unevenness due to inner circuits. When a resist is applied onto such unevenness, this unevenness is reproduced to some extent on the surface of the resist. In this case, best focus is not always available all over the wafer. The depth of focus, which is a focus margin, must be sufficiently deep for ultra-fine processing even in such a case.
In FIG. 2, an inner circuit 22, an intrastratum insulating film 23 and a resist layer 24 are formed over a substrate 21. This drawing illustrates how the resist layer 24 is exposed to light for the formation of a contact hole in the intrastratum insulating film 23, on the supposition that a substrate wafer is exposed to light at three places by moving the stage having the substrate wafer placed thereon.
The resist layer 24 reproduces, on its surface, unevenness of the inner circuits. In this case, a distance between the light source for exposure and the resist layer is not always constant on the whole surface of the wafer. Moreover, the light intensity upon exposure has a predetermined distribution. For example, in spite of the best focus at the site (2), the sites (1) and (3) are under the state of defocus and the intensity of the light incident on the resist inevitably becomes weak. When a conventional resist is employed, exposure is insufficient at the site of weak light intensity, leading to a marked reduction in resolution.
A description was so far made of the lowering in the resolution due to shortage in the light intensity upon exposure. In addition, a film decrease upon alkali development becomes one factor for disturbing ultrafine processing.
FIG. 3 is a schematic view illustrating the etching of the intrastratum insulating film 32 with a resist pattern formed over the intrastratum insulating film 32 laid over the substrate 31. In FIGS. 3(b-1) to 3(b-3), an ordinarily employed resist 33b is used. As is apparent from FIG. 3(b-2), a developer causes a film decrease of the resist 33b, which prevents the formation of a good rectangular pattern. When the intrastratum insulating film is etched using this pattern, the narrowing of the intrastratum insulating film occurs. This pattern is therefore not suited for ultrafine processing.
An object of the present invention is therefore to provide a chemically amplified resist composition which permits ultrafine processing improved in the depth of focus and is excellent in the pattern rectangularity, in consideration of the above-described problems.
In a first aspect of the present invention, there is thus provided a chemically amplified resist composition comprising an photoacid generator which releases an acid by exposure to light and an acid sensitive resin which has an alkali soluble group protected with a dissolution controlling group and is converted into an alkali soluble resin by the cleavage of the dissolution controlling group caused by the action of the acid, wherein the acid contains a sulfonic acid group and a carboxyl group and the alkali soluble resin contains a carboxyl group.
In the above-described chemically amplified resist composition, the photoacid generator is preferably a compound represented by the following formula (1):
R1(CO)2Nxe2x80x94OSO2xe2x80x94R2xe2x80x94COOC(CH3)3xe2x80x83xe2x80x83(1)
wherein R1 represents a dicarboxyimide compound residue and R2 represents a cyclohexylene or phenylene group.
In the above-described chemically amplified resist composition, the acid sensitive resin is preferably represented by the below-described formula (2) or (3) and has a weight-average molecular weight of 3,000 to 30,000: 
wherein R3 represents a tert-butyl group, tetrahydropyranyl group or R4(R5O)CHxe2x80x94 in which R4 and R5 each independently represents a C1-4 alkyl group, x stands for 0.4 to 0.9 and y stands for 0.1 to 0.9.
In the above-described chemically amplified resist composition, the photoacid generator is incorporated in an amount of 1 to 15 wt. % relative to the acid sensitive resin.
In a second aspect of the present invention, there is also provided a chemically amplified resist composition comprising an photoacid generator which releases an acid by exposure to light, and an acid sensitive resin which has an alkali soluble group protected with a dissolution controlling group and is converted into an alkali soluble resin by the cleavage of the dissolution controlling group caused by the action of the acid, wherein the acid sensitive resin is represented by the below-described formula (4) or (5) and has a weight-average molecular weight of 100,000 to 5,000,000: 
wherein R6 represents a crosslinked structure of xe2x80x94Oxe2x80x94C(CH3)2xe2x80x94Oxe2x80x94 or xe2x80x94COxe2x80x94Oxe2x80x94C(CH3)2xe2x80x94Oxe2x80x94COxe2x80x94, R8 represents a hydroxyl group or a carboxyl group, z stands for 0.1 to 0.9 and w stands for 0.1 to 0.9.
In the chemically amplified resist composition, when the crosslinked structure R6 of the acid sensitive resin is xe2x80x94COxe2x80x94Oxe2x80x94C(CH3)2xe2x80x94Oxe2x80x94COxe2x80x94, the photoacid generator is preferably a compound represented by the following formula (1):
xe2x80x83R1(CO)2Nxe2x80x94OSO2xe2x80x94R2xe2x80x94COOC(CH3)3xe2x80x83xe2x80x83(1)
wherein R1 represents a dicarboxyimide compound residue and R2 represents a cyclohexylene or phenylene group.
In the chemically amplified resist composition, the photoacid generator is incorporated in an amount of 1 to 15 wt. % relative to the acid sensitive resin.
The chemically amplified resist composition according to the first aspect of the present invention comprising a photoacid generator which releases, by exposure to light, an acid containing both a sulfonic acid group and a carboxyl group; and an acid sensitive resin which undergoes cleavage of a dissolution controlling group by the action of the acid to form an alkali soluble resin containing a carboxyl group has a remarkably improved maximum dissolution rate and also improved focus depth by the association of the sulfonic-acid-containing acid with the alkali soluble resin in an alkali developer.
The chemically amplified resist composition according to the second aspect of the present invention comprises a photoacid generator and an acid sensitive resin which has a huge molecular weight and from which the dissolution controlling group is cleaved by the decomposition of the partial crosslinked structure due to the acid released from the photoacid generator. This chemically amplified resist has a sufficient molecular weight at unexposed regions, which makes it possible to lower the minimum dissolution rate and to form a good rectangular pattern, thereby improving the depth of focus.
Moreover, by using, in combination, the photoacid generator which releases an acid containing both a sulfonic acid group and a carboxyl group by exposure to light, and an acid sensitive resin which forms a carboxyl-containing alkali soluble resin by the decomposition of its partial crosslinked structure caused by the acid, it is possible to simultaneously accomplish an increase of the maximum dissolution rate and decrease of the minimum dissolution rate, whereby the excellent focal depth is available.
Use of such a chemically amplified resist composition having a deep focal depth makes it possible to conduct ultrafine processing such as formation of a contact hole of 0.20 xcexcm or less or formation of a circuit having a line spacing or line width of 0.15 xcexcm.